With respect to a semiconductor device, especially a silicon device, integration and reduction in power consumption have been achieved and miniaturization follows Moore's law in which an integration degree increases four times every three years. However, in recent years, the gate length of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is reduced to 20 nm or less and the cost of a lithography process is greatly increased. Namely, the price of a lithography apparatus and the price of a mask set are remarkably increased. Further, it becomes difficult to achieve integration and miniaturization of the semiconductor device according to the scaling rule that follows Moore's law because of the physical limitation of the size of the device, in other words, because of the limitation of operation or the limitation of variation. Accordingly, it is necessary to improve the performance of the device by another approach different from the scaling rule.
In recent years, a reconfigurable programmable logic device called an FPGA (Field-Programmable Gate Array) that is classified between a gate array and a standard cell has been developing. The FPGA can be arbitrarily programmed by a customer himself after a chip is manufactured. Namely, the FPGA includes resistive-change elements inside the multilayer wiring layer and the customer himself can arbitrarily connect the wiring electrically. By using the semiconductor device mounting such FPGA, the flexibility of the circuit can be improved.
The resistive-change element is used for a MRAM (Magnetoresistive Random Access Memory), a PRAM (Phase-change Random Access Memory), a ReRAM (Resistance Random Access Memory), a CBRAM (Conductive Bridge Random Access Memory), or the like.
The resistive-change element used for the ReRAM among these memories includes two electrodes and a resistive-change film made of metal oxide sandwiched between these electrodes and a property in which a resistance value changes when an electric field is applied between two electrodes is used. Namely, by applying the electric field between two electrodes, a filament is formed inside the resistive-change film and whereby, a conductive path is formed between two electrodes and the resistance between two electrodes is reduced. This state is defined as an ON state. On the other hand, by applying the electric field whose polarity is opposite to that of the above-mentioned electric field between two electrodes, the filament disappears and whereby, the conductive path formed between two electrodes disappears and the resistance is increased. This state is defined as an OFF state. By changing the polarity of the applied electric field, the value of the resistance between two electrodes greatly changes. Namely, the state can be changed from the ON state to the OFF state or vice versa and the switching can be performed.
Because the resistance value in the ON state is different from the resistance value in the OFF state, an electric current flowing in the resistive-change element changes according to the state of the resistive-change element. Accordingly, the ReRAM stores data by using this characteristic. When the data is written in the ReRAM, a voltage value, a current value, and a pulse width that are required for changing the state from the ON state to the OFF state or vice versa are selected and applied according to the data to be stored. By this operation, the filament is formed or lost, in other words, the conductive path is formed or lost.
A kind of resistive-change element used for the ReRAM uses metal ion movement in an ion conductor, metal deposition caused by reduction of a metal ion in an electrochemical reaction, and metal ion formation caused by oxidation of metal. In non-patent literature 1, there is disclosed a nonvolatile switching element in which the value of the resistance between the electrodes between which the resistive-change film is sandwiched is reversibly changed. A RAM using this nonvolatile switching element is called a CBRAM.
The nonvolatile switching element disclosed in non-patent literature 1 is composed of a solid electrolyte consisting of the ion conductor and a first electrode and a second electrode that are provided so that the electrodes contact with each of two surfaces of the solid electrolyte. A standard formation Gibbs energy ΔG in a process in which a metal ion is formed by the oxidation of a first metal of which the first electrode is composed is different from a standard formation Gibbs energy ΔG in a process in which a metal ion is formed by the oxidation of a second metal of which the second electrode is composed. The first metal of which the first electrode is composed and the second metal of which the second electrode is composed that are described in non-patent literature 1 are selected as follows.
First, when a voltage for changing the state from the OFF state to the ON state is applied between the first electrode and the second electrode, the first metal of which the first electrode is composed is oxidized by electrochemical reaction induced by the applied voltage and the metal ion is formed at a boundary face between the first electrode and the solid electrolyte. At this time, the metal that can be supplied in the solid electrolyte as the metal ion is selected as the first electrode.
On the other hand, when a voltage for changing the state from the ON state to the OFF state is applied between the first electrode and the second electrode, the first metal is oxidized by electrochemical reaction induced by the applied voltage and the metal ion is formed when the first metal is deposited on the surface of the second electrode. At this time, the first metal is melted in the solid electrolyte as the metal ion. On the other hand, a metal which is not oxidized by the applied voltage and does not form the metal ion is selected as the second metal of which the second electrode is composed.
Switching operation of a metal-bridge-type resistive-change element in which the ON state and the OFF state of the resistive-change element are formed by formation of a metal-bridge-type structure in which the first electrode and the second electrode are bridged by depositing the first metal of which the first electrode is composed on the second electrode and melting of the metal-bridge-type structure will be described.
In a transition process (referred to as a set process) in which the state is changed from the OFF state to the ON state, when the second electrode is grounded and a positive voltage is applied to the first electrode, the metal of the first electrode is oxidized, the metal ion is formed at the boundary face between the first electrode and the solid electrolyte, and the metal melts in the solid electrolyte. On the other hand, in a second electrode side, the metal ion in the solid electrolyte is reduced to the metal and the metal is deposited by an electron supplied from the second electrode. The metal-bridge-type structure is formed in the solid electrolyte by the deposited metal and whereby, the first electrode is electrically connected to the second electrode and the state of the switch is changed to the ON state.
On the other hand, in a transition process (referred to as a reset process) in which the state is changed from the ON state to the OFF state, when the second electrode is grounded and a negative voltage is applied to the first electrode, the metal of which the metal bridge is composed is ionized and the metal is eluted in the solid electrolyte. When the elution proceeds, a part of the metal-bridge is disconnected, the first electrode is electrically disconnected from the second electrode, and the state of the switch is changed to the OFF state.
Further, when the metal bridge is being melted, the bridge becomes thin and whereby, the resistance between the electrodes increases. Further, when the concentration of the metal ion included in the solid electrolyte changes, the relative permittivity of the solid electrolyte changes and whereby, the capacitance between the electrodes changes. After these changes occur, finally, the electrical connection is disconnected.
Further, with respect to the metal-bridge-type resistive-change element whose state is changed to the OFF state, when the second electrode is grounded and a positive voltage is applied to the first electrode again, the transition process (the set process) in which the state is changed from the OFF state to the ON state proceeds. Namely, in the metal-bridge-type resistive-change element, the transition process (the set process) in which the state is changed from the OFF state to the ON state and the transition process (the reset process) in which the state is changed from the ON state to the OFF state can be performed reversibly.